Question

What could cause a gaseous substance to liquify?

Answer 1

Temperatures at or below a critical temperature, `T_c` for that gas, along with higher pressures.

Explanation:

Anytime a substance (solid or gas) is turned into a liquid, the process is called liquefaction. When turning gas into liquid, the distance between gas molecules is large compared to the distance between liquid molecules. So there is a lower density in the gaseous form and a higher density in liquid form. So, when a gas turns into a liquid, it's called condensation.

Ideal gases do not get converted into liquid form. So the gas must be a real gas. Three steps are required for liquefaction:

1. Start with a gas only
2. Vapor and liquid exist in equilibrium (vapor`rightleftharpoons`liquid)
3. End with a liquid only

This process requires varying temperature and pressure at the same time. Neither keeping a fixed temperature and only changing pressure, nor keeping a fixed pressure and only changing temperature will work. Instead, there must be some constraints on both temperature and pressure.

If the temperature is too high, the molecules will be moving too quickly to allow the intermolecular forces of attraction to form the required gas`rightleftharpoons`liquid equilibrium. However, if the temperature is low enough, the molecules will also be moving slow enough for the intermolecular forces of attraction to liquefy.

Similarly, low pressures result in greater distances between the molecules, while higher pressures will result in shorter distances. Thus, a low temperature/high pressure combination will be required to liquefy a gas. Note that if the temperature and pressure conditions are such that gas can liquefy, we call the gas vapor.

Using a graph of isotherms for a gas of interest (e.g., `"CO"_2`), choosing a temperature below the critical temperature for that gas, and then looking in the high pressure region, you can identify a good Temperature / Pressure combo.